In a pressure sensitive adhesive layer used for an optical member, its thickness and surface uniformity are required at high levels. Accordingly, its polymer composition is applied at lower viscosity by reducing a concentration of a polymer. In the present situation, however, an amount of an organic solvent used is desired to reduce to the minimum level for environmental consideration.
For a method of using no organic solvent, there are emulsion pressure sensitive adhesives or UV-polymerizable pressure sensitive adhesives, but use of these adhesives for optical members does not reach a practical level due to a problem in water resistance and uniformity in thickness.
As a method wherein a viscosity of a polymer solution is not increased even if a concentration of a polymer is increased, there is a method of decreasing a molecular weight of the polymer. However, an optical member used in a liquid crystal display etc., for example, a polarizing plate or a retardation film, is attached via an adhesive to a liquid crystal cell, but the optical member expands or shrinks under heating conditions or humid conditions and thus raising or peeling easily occurs after attachment. When the molecular weight of a polymer is low, raising or peeling easily occurs under heating conditions or humid conditions, thus making it difficult to decrease the molecular weight of the polymer.
For the purpose of maintaining excellent adhesive properties even under high-temperature humid conditions to prevent foaming or peeling upon dimensional change, a pressure sensitive adhesive composition for a polarizing plate, which comprises a plurality of specific (meth)acrylic polymers and a crosslinking agent, is disclosed (Patent Document 1).
For the purpose of decreasing a change with time in cohesion and adhesion at a high temperature or at a high temperature under high humidity, a pressure sensitive adhesive composition, which comprises a curing agent and a specific silane compound incorporated into an acrylic resin, is disclosed (Patent Document 2).
However, these conventional pressure sensitive adhesive compositions cannot satisfy both reduction in an amount of an organic solvent used and the high surface uniformity of a pressure sensitive adhesive layer formed.
Patent Document 1: JP-A 2003-49141
Patent Document 2: JP-A 8-199131